In common rail fuel systems for diesel engines, a high pressure fuel pump receives fuel at relatively low pressure (e.g. transfer pressure) from a low pressure pump and pressurises fuel to a high level suitable for injection. The high pressure fuel pump supplies pressurised fuel to a fuel accumulator volume, referred to as the common rail, from where the pressurised fuel is delivered to the downstream fuel injectors of the system.
It is usual to provide the common rail with a pressure limiter valve, or safety valve, which is operable to open in the event that fuel pressure within the common rail exceeds a predetermined safe level (referred to as the threshold pressure level). The pressure limiter valve comes into effect should a failure occur in the common rail fuel system and provides a means by which over-pressurised fuel can escape to low pressure to avoid the risk of rupture and failure of pump parts, or any other component in the common rail fuel system, including, for example, the common rail, high pressure pipes, pressure sensors or injectors.
One known pressure limiter valve arrangement for performing the aforementioned function is shown in FIG. 1. The valve arrangement includes a first valve housing 10 defining an inlet channel 12 and a second valve housing 14 defining an outlet channel 16. The inlet channel 12 communicates with fuel at high pressure within the common rail (not shown) and the outlet channel 16 communicates with a low pressure fuel drain (not shown). A valve member 18 in the form a ball valve is engageable with a valve seating 20 to control communication between the common rail and the low pressure drain. The valve seating 20 is spherical in form and defines a valve seating area which accommodates the seated ball 18. A spring 22 acts on the ball valve 18 through an abutment member 24 which is slidably received in a bore 26 defined by the second valve housing 14, and serves to urge the ball valve 18 against the valve seating 20. The spring 22 acts against fuel pressure within the inlet channel 12 and, hence, against fuel pressure within the common rail.
When in the seated position (i.e. ball valve closed), fuel within the common rail is unable to flow to the low pressure drain past the valve seating 20. For a correctly functioning pump, the ball valve 18 remains seated throughout pump life and the threshold pressure level is never exceeded in the rail. However, in the event of a failure in the common rail fuel system causing an increase in fuel pressure within the common rail above the threshold pressure level, the force acting on the ball valve 18 in an opening direction (i.e. against the spring force) is sufficient to lift the ball valve 18 from the valve seating 20. In such circumstances fuel within the common rail is able to flow to the low pressure drain through the outlet passage 16, thus relieving high fuel pressure within the rail. The threshold pressure level at which the valve opens is determined by the ratio of the spring force to the area of the seating 20. Another pressure limiter is known from US2008/0047621.
If the threshold pressure is exceeded, causing the ball valve 18 to lift, a restriction is created between the ball valve 18 and the valve seating 20. The pressure required to keep the ball valve open is equal to the spring force divided by the new surface area over which the pressure acts. This new surface area is a combination of the exposed surfaces of the ball valve 18, and the abutment member 24, which can be approximated as the cross sectional area of the bore 26. As this area is much larger than the valve seating area, the pressure required to keep the ball valve open, or the regulation pressure, is lower than the pressure required to open it initially. However, due to the restriction between the ball valve 18 and the valve seating 20, the pressure that is exerted over the surfaces of the ball valve 18 and abutment member 24 is considerably lower than the fuel pressure within the common rail. Therefore, the regulation pressure for the valve arrangement in FIG. 1 is not much lower than the opening pressure. This effect is illustrated in FIG. 2.
The effect of this is that the fuel flows through the valve arrangement at relatively high pressure, and therefore at a high flow rate. This means that if the ball valve is open for more than a few minutes, damage could occur to the common rail system or to the ball valve itself. Damage may be caused through one of a number of mechanisms: (1) the high pressure and flow rate lead to a very high temperature around the ball valve; (2) a combination of high pressure and high flow rate cause direct erosion of the ball valve and piston; and (3) the high pressure may cause damage by fatigue, creating cracks in the components.
It is desirable in modern diesel engines for common rail systems to offer a “limp home” mode, in which the system is able to comfortably operate for significant periods in the event of a pump failure causing high pressure in the system. Valve arrangements such as that shown in FIG. 1 are not suitable for this requirement, as they can only operate effectively for short periods.
By way of background to the invention, U.S. Pat. No. 5,295,469 to Nippondenso Co., Ltd. discloses a pressure limiter valve which offers a limp home mode. However, the arrangement described in this patent suffers from the drawback that a piston member must be formed with grooves in order to create the limp home effect. The grooves in the piston member also need to communicate with an annular groove in the valve housing and, therefore, the tolerances involved in manufacturing the components of the valve will be relatively tight. As a consequence, the valve arrangement is difficult, and expensive, to manufacture.
It is one object of the present invention to provide an improved valve arrangement, for use in a high pressure fuel system, which overcomes or alleviates the aforementioned disadvantages known in the prior art.